Problem: Solve for $n$ : $-9 = n - 18$
Explanation: Add $18$ to both sides: $ \begin{eqnarray} \\ \hphantom{-9 {+ 18}} &\hphantom{=}& \hphantom{n} \\ -9 &=& n - 18 \\ \\ {+18} && {+18} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -9 {+ 18} &=& n \\ \end{eqnarray} $ Simplify. $ n = 9$